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21.
稀疏角采样与减小X射线源电流可有效降低多能谱CT低辐射剂量,然而会导致投影数据不足且包含较大噪声,重建图像会严重降质。针对这一问题,本文对传统全核变分（TNV）正则化方法进行推广,利用非局部梯度向量构成的雅克比矩阵的低秩特性,提出非局部全核变分（NLTNV）正则化方法。该方法用单个正则项同时建模能谱CT图像的结构相似性、梯度域稀疏性与非局部自相似性3种先验信息,能恢复稀疏角度投影含较大噪声（剂量较低）时图像的结构特征,并且有效缓解了用多正则项建模多能谱CT图像不同先验信息所导致的正则化参数过多问题。此外,基于NLTNV的重建模型为凸优化模型,保证了算法的稳定性与收敛性。实验结果表明,与TNV正则化方法相比,本方法显著提升重建图像的整体质量。 相似文献
22.
Virioplankton dynamics at seven sample sites (from two freshwater lakes) with different trophic states were monitored over
the period of a year. Water was sampled monthly from August 2004 to July 2005, and the abundances of viruses and bacteria
were measured using direct epifluorescence microscopy counting. Results indicated that both natural and anthropogenic factors
could influence the distribution of virioplankton. Temporally, viral abundance was significantly correlated to bacterial density
and water temperature. Spatially, viral abundance was significantly correlated to trophic state. This in turn indicated that
viral abundance was directly dependant on host abundance in eutrophic lakes, while trophic state and temperature could drive
the distribution of virioplankton. The virus-bacteria ratio was significantly lower in less productive water-bodies. The result
implied that the control of virioplankton on their hosts may change according to the host density. 相似文献